The present invention relates to minute field-emission elements, which are capable of integration and operable at a low voltage. The present invention relates also to the methods of fabricating the minute field-emission elements.
This application is related to patent application Ser. No. 08/269,676 filed Jul. 1, 1994, by the same inventor as an inventor of the present invention.
At the outset, it is to be noted that the following description and the claims use "( )" to describe a crystal plane and "&lt;&gt;" to describe a crystal direction. Such usages are well known in crystallography.
The fabrication of miniaturized field-emission elements became possible by the advancements of semiconductor fabrication technologies. In particular, Spindt et al. disclosed the fabrication of a cone-shaped field-emission cathode. (C. A. Spindt, J. Appl. Phys, Vol. 47, p. 5248 (1976).
The conventional method of fabricating a field-emission cathode disclosed by Spindt et al. is shown in FIGS. 13(a)-13(d) and is explained below.
As shown in FIG. 13(a), the fabrication process is begun with depositions of an insulation layer 402 and a metal layer 403 utilized as a gate electrode on a conductive substrate (silicon) 401. A round small hole 404 is then formed in said metal layer 403 and insulation layer 402 by using a conventional photolithographic process. Next, as shown in FIG. 13(b), a sacrificing layer 405, made of a material such as alumina, is vacuum deposited on the substrate 401 at a shallow angle thereto and the gate electrode. As a result, the diameter of gate aperture 404 is substantially reduced. Then, as shown in FIG. 13(c), the metal layer 406, made of a material such as molybdenum, is deposited perpendicularly to the substrate 401. The gate-aperture diameter is gradually reduced as the metal layer 406 is vacuum deposited, and a cone-shaped emitter (cathode) 407 is formed within gate aperture 404 since the gate-aperture becomes smaller as the deposition proceeds.
Lastly, as shown in FIG. 13(d), the fabrication process is completed by removing the sacrificing layer 405 and the unnecessary metal layer 406 using an etching or lift-off method. The field-emission cathode 407, thus obtained, is operable by applying a high-voltage on gate electrode 403. This causes electrons to be drawn into a vacuum from the cathode 407. The electrons are collected by an anode (not shown) disposed at a position opposing the cathode 407.
Another process for fabricating a cone-shaped field-emission cathode was disclosed by Gray et al. (H. F. Gray et al., IEDM Tech. Dig. P. 776 (1980)). The conventional method of fabricating a field-emission cathode, using a silicon substrate and anisotropic etching, is shown in FIGS. 14(a)-14(e) and is explained below.
As shown in FIG. 14(a), the fabrication process is begun with the deposition of a silicon oxide film 412 on the (100) plane surface of a conductive (silicon) substrate 411. Then, as shown in FIG. 14(b), a photolithographic process is applied to the film 412 to form a circular mask 413. Next, as shown in FIG. 14(c), part of the silicon substrate under the mask 413 is formed into a cone 414 having a sharp top 417 (FIG. 14(e)), by using anisotropic etching to slowly etch the (111) crystal plane in a slanted relationship with the (100) plane surface of the silicon substrate 411. Next, as shown in FIG. 14(d), an insulating layer 415 and a metal layer forming a gate electrode 416 are deposited around the cone 414. The circular mask 413 prevents the insulating layer 415 and the gate electrode 416 from forming on the side or slanted surface of the cone 414. Lastly, as shown in FIG. 14(e), the mask 413 and the insulating and metal layers thereon, are removed, resulting in a field-emission cathode 417 having a cone shape.
It is possible to fabricate a field-emission cathode having a sharper top than the cone-shaped cathodes disclosed by Spindt et al. and Gray et al. Betui discloses a process for fabricating a field-emission cathode using a combination of dry etching silicon and thermal oxidation. (K. Betui, Tech. Digest IVMC '91, 26 (Nagnhama 1991)). The conventional method of fabricating a field-emission cathode, as disclosed by Betui, is shown in FIGS. 15(a)-15(e) and is explained below.
As shown in FIG. 15(a), the fabrication process is begun by forming a silicon oxide film 432 on a silicon substrate 431. Next, as shown in FIG. 15(b), a photolithographic process is applied to the silicon oxide film 432 to form a circular mask 433. Next, as shown in FIG. 15(c), dry etching with appropriate conditions is used to form a protrusion 434 of a cylindrical shape under the mask 433. Then, thermal oxidation is applied to form a silicon oxide film 435 leaving the protrusion 434 with a sharp apex 436. Next, as shown in FIG. 15(d), an insulating film 437 and a metal film 438, to be used as a gate, are deposited around the cathode 436. Lastly, as shown in FIG. 15(e), the mask 433 and the insulating and metal films thereon are removed, resulting in a field-emission cathode 436 having a sharp apex.
It should be noted that albeit, the conventional processes described above refer to cone-shaped cathodes, the processes can also be used to fabricate cathodes having a pyramid form.
In forming the cone-shaped cathodes described above, it is preferable that the diameter of the hole of the gate electrode be as small as possible, as the size of the hole affects the operable voltage and current density characteristics of a field-emission type electron source. Under the processes described above using a conventional photolithographic process, the smallest hole obtainable is about 1 .mu.m. Smaller holes are possible using electron beams or x-ray lithography. However, such processes are expensive to operate and do not fabricate elements with uniform characteristics.
None of the conventional methods described above disclosed fabricating a tower type field-emission cathode. A tower type cathode, rather than a cone-shaped cathode, produces a stronger electric field and can be operable at lower voltages. However, none of the known field-emission element fabrication processes are capable of re-producing such a field-emission element. The known fabrication processes are not capable of producing field-emission electron sources operable at low working voltages with good reproducibility.